1. Field Of The Invention
This invention relates to data reproducing apparatus and, more particularly, to such apparatus which is capable of reproducing data that had been recorded on a magnetic medium by one of plural types of modulation, such as biphase-mark modulation or 8-10 modulation.
2. Description Of The Prior Art
In one type of data recording system known to those of ordinary skill in the art, audio signals are PCM-encoded, time-base compressed and recorded on a magnetic medium. For example, in a video tape recorder (VTR) of the 8 mm format, the time-compressed PCM-encoded audio signal is modulated and recorded in a portion of the usual slant tracks on magnetic tape. One example of such recording is described in U.S. Pat. No. 4,551,771, assigned to the assignee of the instant invention. A particular type of modulation used to record the time-compressed, PCM-encoded audio signal is biphase-mark modulation.
It has been found desirable to record the aforementioned biphase-mark modulated audio signal in a portion of each of the adjacent slant tracks, with the remainder of each track having video information recorded therein. Typically, two magnetic heads are used to record respective tracks alternately on the tape. Of course, during playback, a head first reproduces the biphase-mark modulated audio signal and then, as the head continues to scan a track, the video signal is reproduced. As described in the aforementioned patent, when one head reproduces the audio portion of the slant track, the other head is in its final phase of reproducing the video portion from an adjacent track.
A conventional audio signal reproducing system used in one type of VTR, such as the aforementioned 8 mm VTR, regenerates a clock signal CK from the biphase-mark modulated audio signal reproduced from the magnetic tape. This regenerated clock signal is used to demodulate the PCM-encoded audio information, thereby recovering the original audio signal for sound reproduction. In a preferred type of clock regeneration circuit a phase locked loop is used to generate a "clean" clock signal that is phase locked to the clock signal component included in the reproduced biphase-mark modulated signal, and also is substantially equal in frequency to that clock component. This clock regeneration circuit and PCM decoder function satisfactorily to recover the audio signals which had been recorded in typical 8 mm VTR format.
Recently, it has been proposed to modify the audio signal modulation technique used in 8 mm VTR recording. One such proposal has suggested that the modulation technique used in rotary head digital audio tape recorders (R-DAT) be used in place of the biphase-mark modulation technique. The R-DAT modulation, although relying on PCM encoding, records audio information with a considerably higher bit rate than is used in biphase-mark modulation. A so-called 8-10 modulation technique, described in U.S. Pat. Nos. 4,617,552 and 4,577,180, both assigned to the assignee of the present invention, has been proposed for 8 mm VTR recording. In the 8-10 modulation technique, an 8-bit signal, such as a PCM signal, is converted into a 10-bit signal and recorded. By using 10 bits, a large number of successive binary "1"s and "0"s can be avoided. This is desired because successive "1"s and "0"s introduce an unwanted DC component to the reproduced digital data. By avoiding or at least minimizing this DC component, clock regeneration can be carried out accurately.
However, it has been found that biphase-mark modulation (referred to simply as a first type of modulation) and 8-10 modulation (referred to simply as a second type of modulation) record information in different frequency bands. Furthermore, these two types of modulation techniques are accompanied by a large difference in their respective sampling or clock frequencies. For example, the sampling/clock frequency used in the first modulation technique (i.e. in biphase-mark modulation) is on the order of about 11.6 MHz, whereas the sampling/clock frequency used in the second modulation technique (i.e. in 8-10 modulation) is on the order of 14.8 MHz. Consequently, systems using these two different types of modulation require different Nyquist conditions for converting electronic signals to magnetic format and they also exhibit different noise distribution spectra. As a result, if one clock regeneration circuit is used to reproduce audio signals which are recorded on magnetic tape in accordance with one modulation technique, the bit-error rate of the demodulated PCM-encoded data is increased when audio information recorded by the other modulation technique is reproduced. That is, the bit-error rate is increased if the clock regeneration circuit normally used to recover biphase-mark modulated signals is used to reproduce 8-10 modulated signals, and vice versa.